1. Field of the Invention
The present invention pertains to devices for measuring the air or gas flow rates through conduits especially at low flow rates, and more particularly, it pertains to head flowmeters or air flow measuring devices of the type which present a restricted orifice or a plurality of restricted orifices to the air flow and measure the pressure drop thereacross in order to determine the flow rate.
2. Description of the Prior Art
In head flowmeters, a device designed to introduce a slight resistance to the flow of air or gas is placed within the conduit through which the air is flowing. The difference between the pressures appearing at the wall of the conduit sections upstream and downstream of the flow impeding device is measured, and the magnitude of that pressure difference is directly utilized to determine the amount of air (in standard cubic feet per hour, SCFH, for example) flowing through the conduit (i.e., the greater the pressure differential, the greater the flow). In its simplest form, the flow impeding device is a thin plate mounted perpendicularly to the axis of the pipe, in such a manner as to prevent the flow of air except such air as may flow through an orifice machined in the plate and located on the centerline of the conduit.
Air flow measuring devices of this type respond in accordance with the law of conservation of energy. In its most general form the air flow in the region of the aperture is described mathematically by Bernoulli's equation, which contains a term for the kinetic energy of the fluid flowing toward the orifice and a second term for the kinetic energy of the fluid flowing away from the orifice. The kinetic energies of the fluid, as expressed in this equation, are each defined by a singles term containing the square of the stream velocity. In practice, the relationship between the kinetic energy of the fluid moving along the centerline of the stream and the kinetic energy of the entire stream is complicated by three factors:
first, if the flow in the conduit is smooth the distribution of velocity across the conduit varies with flow velocity, becoming parabolic (i.e., greatest at the centerline of the conduit at low stream velocities and rather flat (i.e., uniform across the width of the conduit except directly adjacent to the conduit wall) at high velocities, and as a result, the relationship between the pressures measured at the conduit wall and the pressure difference caused by the aperture becomes a function of the stream velocity;
second, the equation assumes that all of the kinetic energy of the fluid is contained in motions directed along smooth pathways and not lost in motions with randomized directions as caused by turbulence (i.e., the effect of the orifice on the measured pressure difference is influenced by the extent of the turbulence in the stream); and,
third, the equation does not consider the effects of the momentum of the stream discharged from the orifice. With regard to the latter point, at low velocities, the momentum of a small volume of air is low and allows the air to respond to the radial distributions of pressure within the conduit while traveling a short distance in the direction of the conduit axis. At high velocities, the momentum of that small volume is larger and carries the small volume further in the direction of the conduit axis in comparison with its movement perpendicular to the conduit axis caused by the radial distribution of pressures within the conduit.
As a result of these limitations, the configuration of the airflow measuring device in prior art devices has been altered
(1) by carefully shaping the edges of the orifice in the plate in an effort to reduce the amount of turbulence created by the orifice itself;
(2) by replacing the orifice with a nozzle carefully shaped to prevent introducing turbulence in either or both of the upstream and downstream portions of the pipe;
(3) by placement of the wall taps used to measure the pressures at locations carefully selected to minimize the variability of the measured pressures caused by variations in stream momentum or by turbulence caused by the head flowmeter itself;
(4) by introducing multiple aperture devices (such as screens, for example) which break up the air flow at the restriction into a plurality of smaller parallel air streams;
(5) by introducing devices (such as screens, for example) upstream of the flowmeter that are designed to reduce turbulence in the incident stream; and,
(6) by correcting the readings of the measuring device by use of a "discharge factor" obtained by careful calibration of the device under different conditions of flow, and by using that discharge factor to provide an empirical correction of the measured data on the assumption that turbulence and other unmeasured properties of the air flow at the time of use are the same as those existing at the time of calibration.
Prior art air flow measuring devices of the foregoing type have used typical screens of the woven mesh type to provide the plurality of restricted orifices across the air flow path, such devices being shown, for example, in U.S. Pat. No. 3,504,542 to Blevins, U.S. Pat. No. 3,626,755 to Rudolph, U.S. Pat. No. 3,797,479 to Graham and U.S. Pat. No. 5,357,972 to Norlien. With woven mesh type screens, however, an air flow problem on the upstream side of the screen is presented since the air flow (particularly at low flow rates) tends to follow the undulations of the weave at the mesh openings. This results in a tendency to create a circular air pattern about each of the openings in the mesh which vary in accordance with the air flow velocities and thereby hinder accurate readings of flow rate over a suitable range.
Other prior art devices have provided elongated tubular configurations (such as those shown in U.S. Pat. No. 3,071,160 to Weichbrod and U.S. Pat. No. 3,838,598 to Tompkins) for the flow restriction, which for various reasons have not proven to be wholly satisfactory.
Finally, flat plate restrictors have been used with multiple spaced orifices such as shown, for example, in U.S. Pat. No. 5,722,417 to Garbe and U.S. Pat. No. 3,129,587 to Hallanger. Such devices have not proven to be easily manufacturable and readily adaptable for use in various environments, such as in the measuring of relatively low air flow in underground conduits for pressurization of telephone cables or the like.
It will also be noted that the measurement of the pressures at the upstream and downstream sides of the restriction will be affected by where (circumferentially) the pressure is measured particularly if there is an uneven velocity distribution across the conduit as, for example, will be the case if the flow measuring device is installed just downstream of the bend in the conduit. Some prior art devices attempted to remedy this problem by providing a plurality of spaced sampling points for determining the upstream and downstream pressures such as shown by the linear tubes extending across the flow path in the aforementioned U.S. Pat. No. 3,129,587 to Hallanger and by the circumferentially spaced parts in the tubular screen holders in the aforementioned U.S. Pat. No. 3,504,542 to Blevins.